Lawn fungus shows up as distinct patches, spots, or discolorations in your grass that follow recognizable patterns depending on the type of fungal disease. Some create neat circles of dead turf, others coat individual blades with powdery residue or colored pustules, and a few produce visible cottony growth you can see in the early morning. Knowing what to look for on both the overall lawn and individual grass blades helps you figure out which disease you’re dealing with and whether the problem is fungal at all.
Brown Patch
Brown patch is one of the most common lawn diseases, and it looks exactly like its name suggests: roughly circular areas of brown, thinning grass. The patches can range from a few inches to several feet across. What sets brown patch apart is a darker “smoke ring” border that sometimes appears around the edge of the patch, especially in the morning when dew is present. Individual grass blades look water-soaked and matted down, eventually drying to a light brown.
This disease thrives when temperatures hit 80 to 85°F and leaves stay wet overnight. If you’re seeing these patches appear during humid summer weather, particularly in areas where you’ve been fertilizing heavily, brown patch is a strong possibility.
Dollar Spot
Dollar spot starts small. You’ll notice bleached, roughly circular patches from a few inches to about 6 inches across, about the size of a silver dollar (hence the name). Left unchecked, these spots merge into larger, irregular dead areas that can make your lawn look ragged.
The real giveaway is on the grass blades themselves. Each affected blade develops a lesion with a distinctive hourglass or pinched shape, straw-brown in the center with darker reddish-brown borders that stretch across the width of the blade. That pinched shape within the lesion is unique to dollar spot and the most reliable way to confirm it. Leaf tips may also die back and turn tan. Dollar spot has a wide active range, appearing when temperatures are anywhere from 60 to 85°F, often during periods of heavy morning dew and high humidity.
Rust
Rust is one of the easiest lawn diseases to identify because it literally rubs off on your shoes. Walk through an affected area and your footwear picks up an orange, yellow, or reddish-brown powder. The disease starts as tiny yellow specks on grass blades, which develop into raised pustules that turn orange, orange-brown, or red. From a distance, an infected section of lawn takes on an overall reddish, brown, or yellowish cast that looks nothing like normal drought stress.
Rust tends to appear in late summer and fall when temperatures are 85 to 95°F, particularly on lawns that are slightly drought-stressed with frequent light rain or heavy dew. Severe infections can kill entire leaf blades, but most cases are cosmetic and the grass recovers once growing conditions improve.
Snow Mold
If you live in a cold climate, you may discover circular patches of dead, matted grass as snow melts in late winter or early spring. These are snow molds, and there are two main types with slightly different appearances.
Pink snow mold produces patches 2 to 10 inches across with pink, white, or tan matted leaf blades in the center, often surrounded by an outer ring of copper-colored grass. The pink coloration comes from fungal growth and tiny spore-bearing structures on the blades. You may also notice white, fluffy fungal threads along the edges of patches, particularly near receding snow or during humid weather. Gray snow mold looks similar in size and shape but lacks the pink tint, instead showing grayish-white fungal growth. Gray snow mold requires actual snow cover to develop, while pink snow mold can appear anytime temperatures sit between 32 and 45°F with enough moisture.
Powdery Mildew and Gray Leaf Spot
Powdery mildew looks like someone dusted your grass blades with flour. A white or grayish powdery coating develops on the upper surface of blades, most commonly in shaded areas during spring and fall when temperatures hover around 65 to 70°F. Cool, damp nights with high humidity are the classic trigger. Heavily shaded lawns under trees are particularly vulnerable.
Gray leaf spot is more dramatic. It creates elongated, gray to brown lesions on grass blades with distinct dark borders, and in hot, humid conditions (80 to 90°F) it can spread rapidly. Affected grass looks scorched and thins out quickly. This disease is especially aggressive on certain grass types and during prolonged periods of high humidity.
Pythium Blight
Pythium blight is the one that looks alarming. It appears suddenly during the hottest, wettest weather of summer, when temperatures reach 85 to 95°F and the soil is waterlogged. You’ll see irregular streaks or patches of dark, water-soaked grass that looks greasy. In the early morning, affected areas may be covered in white, cottony fungal growth that disappears as the day warms up. The streaks often follow drainage patterns or the direction of water flow across the lawn. Pythium moves fast, sometimes killing grass within 24 hours, so it tends to catch people off guard.
How to Tell Fungus From Other Damage
Not every brown patch in your lawn is a fungal disease. Pet urine, drought stress, and insect damage can all mimic the look of fungal infections, so it helps to know the differences.
Pet urine damage creates a very specific pattern: a small crater of brown, dead grass (typically 3 to 6 inches across) surrounded by a ring of unusually dark green, lush growth 6 to 12 inches in diameter. That bright green halo happens because urine contains nitrogen, which acts as fertilizer at lower concentrations around the edges. The dead center goes from gray-green to light brown rapidly, sometimes within 24 hours. The key distinction is that urine-damaged blades don’t appear water-soaked or matted the way fungal infections do, and you won’t find any cottony fungal growth.
Drought stress tends to affect the entire lawn unevenly rather than creating the distinct circular patches typical of most fungal diseases. Grass under drought stress turns a dull blue-green before fading to brown, and footprints remain visible because the blades lack the moisture to spring back. There’s no pattern of rings, borders, or lesion marks on individual blades.
Insect damage from grubs or chinch bugs also creates brown patches, but if you tug on the grass in a grub-damaged area, it lifts up like a carpet because the roots have been eaten away. Fungal diseases attack the blades and crowns of the grass, so the roots typically stay anchored.
What to Check on Individual Blades
Zooming in on individual grass blades is often the fastest way to confirm a fungal problem. Healthy grass blades are uniformly green with clean edges. Fungal infections produce specific markings: lesions with distinct borders (often darker brown or reddish-brown surrounding a lighter center), colored pustules that rub off on your fingers, powdery coatings, or water-soaked areas that look translucent or greasy.
Pull a few blades from the edge of a suspicious patch and look closely. Dollar spot’s hourglass-shaped lesion is unmistakable once you know what to look for. Rust pustules pop open and release colored spores when you rub them. Brown patch causes blades to look wilted and water-soaked at the base before drying out. If the blades just look dry and brown with no distinct lesion patterns, borders, or discoloration, the problem is more likely environmental than fungal.
When Each Disease Typically Appears
Timing matters because different fungi thrive in very different conditions. Hot, humid summer weather (80°F and above) is prime time for brown patch, Pythium blight, anthracnose, and gray leaf spot. Dollar spot has the widest window, appearing anywhere from late spring through early fall whenever humidity is high. Rust peaks in late summer and early fall. Powdery mildew prefers the cooler, damper conditions of spring and fall. Snow molds are strictly cold-weather diseases, developing under snow cover or near-freezing temperatures.
If you’re seeing patches in July during a heat wave, you can immediately narrow the list to warm-weather diseases. Patches appearing in October or after snowmelt point to an entirely different set of fungi. Matching what you see to the current weather conditions is one of the most practical ways to identify what’s happening in your lawn.